With wide application of micro-electrical equipment, especially the mobile communication equipment, charging device for battery becomes the necessity of the mobile equipment. A high-frequency circuit is usually used to design and build a charger for convenient carriage and self-adaptation to the AC100V˜240V mains voltage; therefore, the safety performance of the charger becomes particularly important. A current-limiting resistor against over-current and over-temperature is the key component for the safety of the high-frequency circuit. The present invention provides to meet the demand of safety requirements, further achieving reliability and quick response.
Although the wirewound resistor also has an over-current fuse function, the resistor wire is applied with a high melting point alloy and the alloy wire of the wirewound resistor will melt to realize fuse function, only if subjected to a power which is over 20 times of the rated power. However, in actual applications, when the load is abnormal, the current of the wirewound resistor is often unable to reach the current level, where the resistance wire of the resistor can cut off, and causes the fuse function of the wirewound resistor to not be realized, while the temperature of the wirewound resistor reaches 300° C.˜500° C. or even much higher. This is a serious problem and a dangerous condition for the charger since shell of the charger melts and more seriously a fire occurs when being placed under such high temperature. Under these conditions, people use an external contact type thermal fuse connected in series and placed inside a ceramic box, and when the thermal fuse senses that the temperature of the wirewound resistor reaches the rated temperature of the thermal fuse, the thermal fuse will melt to cut off the circuit. However, thermal fuse occupies additional area in the PCB and it needs 4 bonding pads under such operation.
Moreover, for the safety considerations, the micro-heating elements used in daily life, such as aromatherapy diffuser or mosquito repellant electric liquid vaporizer, are applied with a thermal fuse against over-heating. Existing assembly method is to connect a resistor and a thermal fuse in series, then assemble the unit inside a ceramic box, and the box is filled with solidifiable insulating material. This increases the size of the product; therefore, the heat may be lost and the energy may be wasted.
In addition, the current of the motor of a power tool or an electrical fan is six times the normal working current when they are blocked, under which condition the motor heats quickly. It needs a thermal fuse to cut off the current to prevent a fire because of over-heating condition, but it is not expected to decrease the operation temperature of the thermal fuse to increase the agility. However, mild overload or voltage pulsation happens when the motor works. Under these mild conditions, the thermal fuse is expected not to be cut off so there is an issue with setting up the temperature of the thermal fuse.
Moreover, existing insulation coating uses brittle material to encapsulate the whole device. The device is commonly used in the switch-mode power supply (hereinafter referred to as SMPS), and when explosion occurs due to a shortcircuit of the electric components connected thereafter, the brittle material splashes all over the circuit board.
The structure of a component comprising a thermal fuse, a resistor and protective casing of new, small size, where an integrated structure and fast installation is provided, which solves above problems.